The following invention relates to a method and apparatus for energizing the scanning electrodes of a matrix-addressed AC TFEL panel in a way that conserves energy and allows for the use of larger panels.
Conventional AC matrix-addressed TFEL panels include a set of scanning electrodes and a set of data electrodes sandwiching an electroluminesscent (EL) phosphor laminate. In general, selected data electrodes are energized as the scanning electrodes are energized a line at a time. Once all of the scanning electrodes have been energized in this fashion, a frame of data has been completed. Usually the scanning electrodes are arranged as row electrodes extending from right to left and all are connected to a power source which comprises a high current power IC to provide the scanning voltage. The scanning voltage is selectively gated onto each individual row electrode by individual row driver transistors which act as switches under the control of logic circuits. An example of the way in which the electrodes of this type are conventionally arranged is shown in the U.S. Patent to Kinoshita, et al., U.S. Pat. No. 4,485,379.
An improvement to such conventional TFEL panels is shown in Dolinar, et al., U.S. Pat. No. 4,739,320 which is assigned to the assignee of the present application. The Dolinar, et al. patent discloses a split screen architecture for matrix-addressed AC TFEL panels in which the column drivers are split into top and bottom segments which are separately driven. This enables the row drivers to drive top and bottom row electrodes simultaneously. This conserves energy in the panel and provides for a higher frame repetition rate because of the time savings realized by scanning the top and bottom portions of the screen simultaneously. A drawback to the design of Dolinar, et al. is that the row drivers are driven with the same polarity both in the top and bottom sectors of the screen simultaneously. This places demands on the power supply powering the row driver IC's, and the result is that the power supplies must be made physically large to accommodate the peak energy requirements. This in turn has negative consequences for the overall size of the panel because long electrodes are required for larger panel sizes resulting in a corresponding need for an increase in the size of the power supply. TFEL panels provide an advantage over conventional cathode ray tube displays because of their compact size. If a very large power supply is required, however, this advantage becomes diminished.
A variation of the Dolinar, et al. driving scheme is shown in Flegal, U.S. Pat. No. 4,733,228 assigned to the same assignee and which is incorporated herein by reference. The Flegal patent discloses a symmetric drive scheme where the scanning electrodes are alternately driven positive and negative on alternating frames. A still further variation of this concept is shown in U.S. patent application Ser. No. 166,417 also assigned to the same assignee and now abandoned. According to this application, the row or scanning electrodes alternate between driving voltages of positive and negative polarity as the rows are scanned from top to bottom and polarities are reversed every frame. The problem with this approach is that if the electrodes are arranged in the manner suggested by the Dolinar, et al. '320 patent, the positive and negative power supplies would still be called upon to deliver twice the power for every row electrode that is scanned because the electrodes are connected to the power supply in complementary top and bottom pairs.
A desirable object in the design of such panels would be to utilize split screen architecture along with line by line symmetric drive while at the same time decreasing the overall power requirements for the scanning electrodes, thus permitting a more compact electronics package. This would also enable larger panels to be fabricated without an attendant increase in the size of the power supplies.